Circuit arrangements of this generic type are used, for example, for projection devices which, furthermore, have a color wheel which rotates at a specific frequency. This frequency is normally coupled to the video frequency, which is 48 images per second in Europe and is 63 images per second in the United States. The color wheel is normally rotated at two to four times the video frequency, correspondingly at about 96 to 252 revolutions per second in the case of said examples.
This procedure is illustrated in FIG. 1. In this case, FIG. 1c shows the period duration T. According to the train of curves 1a, a period duration T in the present case comprises three revolutions of the color wheel, with the color sequences R for red, G for green and B for blue being repeated three times in the present case. Color segment changes may be considered as commutation times. In this case, the so-called light valve is switched off briefly in order that no two mixed colors are created in the projection. This switched-off time is used for commutation. The train of curves 1b indicates the actually chosen commutation times. The lamp current is commutated. In this case, two commutation processes are required for one full current period.
The sequence of commutation times shown in FIG. 1b is repeated periodically. This results on average in a commutation frequency of 3 times the video image sequence, in the exemplary embodiment, accordingly, 3×48 Hz=144 Hz for the European video image sequence. If two commutation processes are now required for one complete current period, this accordingly results in a lamp operating frequency of 144 Hz/2=72 Hz. This therefore results in a proportionality factor of 72 Hz/48 Hz=1.5 with respect to the video frequency.
In the event of an increase to four color wheel revolutions per period duration T, this accordingly results in a proportionality factor of 2. The lamp operating frequency is accordingly coupled to the video frequency via this proportionality factor. In this case, however, each lamp has its own optimum lamp operating frequency. This allows operation without flickering, the formation of a stable arc, and high stability of the electrodes throughout their life.
This results in two problems: even if a projection device in which a circuit arrangement such as this is used is designed optimally for European ratios, that is to say the lamp is operated at its optimum lamp operating frequency, then a system such as this is not suitably designed for operation in the United States. On the other hand, only fixed proportionality factors can be set. If an optimum lamp operating frequency with a proportionality factor of 1.5 were now to result, then this could not be achieved by the means from the prior art.